extragalactic reference frame

An extragalactic reference frame is a reference frame tied to remote extragalactic objects, such as quasars.
Such a coordinate system forms a true inertial
reference frame because the reference objects are at such great distances
that their motions across the sky are undetectable. The extragalactic reference
frame that has been adopted by the astronomical community is known as the International Celestial Reference Frame (ICRF).